Pitch actuators are used to adjust the pitch or angle of rotor blades in order to adjust aerodynamic efficiency and, for example the rotor speed, of the wind turbine rotor.
Thus, by adjusting the pitch it is possible to reduce the rotor speed and, thereby, avoid too high loads of components of the wind turbine for example at high wind speeds. However, actuation of the pitch actuator, which is often a hydraulic actuator, naturally reduces life time of the pitch actuator and, thereby, increases the risk for failures. Operational reliability of a wind turbine is of most importance in order to increase the turbine overall power production efficiency and, therefore, it is desirable to increase the service time or reliability of pitch actuators.
U.S. Pat. No. 4,161,658 discloses a wind turbine having a wind driven rotor with a plurality of variable pitch angle blades. The blade angle is scheduled during acceleration and deceleration of the wind turbine by open loop controls to minimize stresses, and is scheduled during powered operation by closed loop controls to maintain desired torque or speed. The closed loop controls contain an integrator which produces an integral blade angle control signal. The scheduled blade angle is fed back to the integrators through an integrator tracking network to maintain the integral blade angle control signal at all times within a preselected range relative to the scheduled blade angle.
Whereas U.S. Pat. No. 4,161,658 discloses pitch control for minimising stresses, U.S. Pat. No. 4,161,658 neither discloses increased service time or reliability of the pitch actuators nor actuation of the pitch actuator as a problem. Accordingly, an improved control system capable alleviating the problem of reduced lifetime of pitch actuators due to actuation of the pitch actuator would be desirable.